Diffusion vacuum pump apparatus



TEMPERATURE (F) J. D. COHOON 3,445,859

DIFFUSION VACUUM PUMP APPARATUS Filed July 1, 1965 E @U5O .02 .0: 0s.O(8 .|00 lzweizior:

I GAP I N JWD- 0050032 United States Patent 3,445,859 DIFFUSION VACUUMPUMP APPARATUS James D. Cohoon, Melrose, Mass., assignor to DresserIndustries, Inc., Dallas, Tex., a corporation of Delaware Filed July 1,1965, Ser. No. 468,847 Int. Cl. F22b 29/00; H05b 3/02 US. Cl. 219-275 8Claims ABSTRACT OF THE DISCLOSURE The pump comprises a cylindricalhousing having a stainless steel base plate forming the lower portion oft e pump boiler. A heater plate, which is bolted to the underside of thebase plate, is fabricated from cast iron; and electrical conductors areembedded in grooves in th heater plate. The base plate and heater plateare of different materials therefore there are different degrees ofthermal deflection of the two plate members during operation. Thethickness of the base plate is det rmined by a formula which takes intoconsideration the radius of the base plate, the normal design operatingpressure of the pump, the normal design heat flux passing through theinterface between the heater plate and base plate, and the physicalcharacteristics of the base plate and heat plate materials. The formulaestablishes a base plate thickness which will result in a predetermineddeflection of the base plate, resulting from the pressure differentialacross the base plate, which pressure deflection is just sufficient tocompensate for the difference in the thermal deflection of the plates.The total deflection of the two plates is then the same to provide goodsurface contact over the entire interface area of the plates.

This invention relates generally to diffusion vacuum pumps and moreparticularly to a unique boiler assembly for such pumps.

The operation of diffusion vacuum pumps is generally well known. Apumping fluid is evaporated in a heated boiler of the pump and theresulting vapor directed at supersonic velocity through a nozzle systemto be fully condensed on a cold surface. The vapor stream, while passingbetween the nozzle system and the condensing surface, accepts bydiffusion gas molecules from the system being evacuated and compressesthese molecules into a higher pressure area which normally communicateswith a mechanical backing pump. The liquid produced on the condensingsurface returns to the pump boiler to be reheated and re-evapo-rated.

There exist commercially two common boiler designs for supplyingevaporation inducing heat to the pumping fluid of diffusion vacuumpumps. One arrangement provides heating elements within the pump casingand submerged in the pumping fluid pool so that heat is transferreddirectly from the heating elements to the pumping fluid. The othercommon boiler arrangement incorporates a heating plate attached outsidethe pump casing in intimate contact with the pump boiler bottom. In thistype heat is transferred from the heating plate through the boilerbottom to the pumping fluid.

The latter arrangement, while offering the advantage of mechanicalsimplicity and low cost, suffers the disadvantage of susceptibility toheater plate failure. These failures normally result from overheating ofthe heater plate because of an insufficient heat transfer from theheater plate into the boiler bottom. The failures have become moreprevalent with the advent of modern high performance diffusion pumpswhich require significantly higher heat flux between the heater plateand the boiler bottom.

The object of this invention therefore is to provide a diffusion vacuumpump which utilizes the mechanically Patented May 20, 1969 "ice simpleexternal heater plate, exhibits the high pumping speeds obtainable withhigher heat fluxes, and is not susceptible to excessive incidence ofpump heater burn out.

One feature of this invention is the provision of a diffusion vacuumpump having a boiler base plate and attached external heater plateconstructed of dissimilar materials and wherein the mechanicalcharacteristics of the boiler base plate and contacting heater plate aresuch that during normal operation of the pump the combined thermal andpressure deflection of the boiler base plate will be substantially equalto the thermal deflection of the heater plate.

Another feature of this invention is the provision of a diffusion vacuumpump of the above featured type wherein the boiler base plate iscircular and of such thickness as to be substantially deflected by thepressure differential across the base plate during normal operation ofthe pump thereby permitting an equalization of deflection experienced bythe boiler base plate and external heater plate.

Another feature of this invention is the provision of a diffusion vacuumpump of the above featured types wherein the boiler base plate has auniform thickness which is less than a certain calculable thicknessdependent on the dimensions of the pump, the materials of constructionand the normal pressure and temperature operating conditions for whichthe pump is designed.

Another feature of this invention is the provision of a diffusion vacuumpump of the above featured types wherein the pump is designed for normaloperation with a heat flux across the interface between the heater plateand boiler base plate which is greater than 15,000 B.t.u. per hour persquare foot of interface area.

Another feature of this invention is the provision of a diffusion vacuumpump of the above featured types wherein the thermal expansioncoefficient of the boiler base plate material divided by its thermalconductivity is greater than the thermal expansion coefficient of theheater plate material divided by its thermal conductivity.

Another feature of this invention is the provision of a diffusion vacuumpump of the above featured types wherein the heater plate comprises acircular block having its surface interrupted by indentations andincluding elect-rically conductive coils positioned in and conforming tothe indentations so as to be in close contact with the circular heaterblock.

Another feature of this invention is the provision of a diffusion vacuumpump of the above featured types wherein the boiler base plate isconstructed of stainless steel and the heater plate is constructed ofiron.

These and other features and objects of the present invention willbecome apparent upon a perusal of the following specification taken inconjunction with the accompanying drawings wherein:

FIG. 1 is a partial schematic sectional drawing of a simplifieddiffusion pump according to the present invention;

FIG. 2 is a partial schematic drawing illustrating the thermal mismatchbetween boiler base plate and heater plate which contributes to heaterplate failure; and

FIG. 3 is a diagram which plots for a particular pump the outside facetemperature of the heater plate versus the resulting air gap existingbetween the heater plate and boiler base plate.

Referring now to the drawings, FIG. 1 shows a cylindrical pump housing11 having its bottom end gas tightly closed by the circular base plate12. Supported by the base plate 12 is a jet assembly 13 which ispartially submerged in the pumping fluid bath 14.

Also attached to the base plate 12 and extending therefrom in adirection opposite to the pump housing 11 is the 3 cylindrical skirt 15.Within the skirt is the heater plate 16 which is attached in intimatecontact with the base plate 12. The heater plate 16 is maintained inposition by the securing nuts 17 screwed upon the threaded shanks of thestuds 18 which extend from the base plate 12 through apertures in theheater plate 16.

The bottom surface of the circular block heater plate 16 is indented bya spiral groove which accommodates the electrically conductive heaterwire 21. The heater wire 21 is forced into intimate contact with thesurfaces of the groove 19 by, for example, peenin'g.

During operation of the diffusion pump shown in FIG. 1 an electricalcurrent from a suitable power source (not shown) is conducted throughthe heating wire 21. The resistance heating produced is conducted intothe heater plate 16 and across the interface 22 into the boiler baseplate 12. Heat is subsequently conducted into the pumping fluid pool 14causing evaporation and the well known diffusion pumping effect withinthe pump housing 11.

Referring now to FIG. 2 there is shown an exaggerated illustration ofthe physical phenomena that produces many of the heater plate failuresexperienced in pumps of this type. The heat flux generated by the heaterwires '21 causes the circular base plate 12 and the circular heaterplate 16 to deform spherically. However as shown in FIG. 2 the degreespherical deflection in the base plate 12 and heater plate 16 are notequal. This inequality of deflection can result from various causes. Forexample, in the preferred pump embodiment shown in FIG. 1 the base plate12 is constructed of stainless steel while the heater plate 16 isconstructed of cast iron. Accordingly the two components exhibitdifferent coefficients of thermal conductivity and expansion causingthem to assume spherical shapes of different radii. Also in theembodiment shown the attached pump housing 11 will exert a restoringmoment on the base plate 12 which moment is not exerted on the heaterplate 16.

FIG. 2 illustrates uneven deflection for a hypothetical situation inwhich the heater plate 16 is unbolted or unrestrained. In actualpractice, with the heater plate bolted as shown in FIG. 1, the bolts 18-attempt to prevent the formation of the gap 23 but are stressed beyondtheir yield point. The bolts subsequently yield and a gap is formedreducing the area of contact between the base plate 12 and the heaterplate 16. The heat flux across the reduced contact area in the centerportion of the base plate 12 causes an increase in the sphericaldeflection mismatch between the two plates. This catastrophic elfectwill continue raising the temperature on the face of the heater plate 16until melting and destruction of the heater element occur.

FIG. 3 is a diagram showing for a typical pump the relationship betweenthe temperature on the face of the heater plate 16 and the gap 23between the heater plate 16 and the base plate 12. The pump used had acast iron heater plate 16 and a stainless steel boiler plate 12 with adiameter of about 6 in. The calculated values plotted in the initialportion of the curve of FIG. 3 illustrate the catastrophic effect onheater plate temperature of an increasing air gap 23. It was calculatedthat failure of the heater element for the particular pump would occurif the gap 23 reached a magnitude of about .005 inch. The length of gap23 of course decreases radially toward the center of the plates but thegaps considered were the maximum gaps existing at the circumference ofheater plate 16. Furthermore the forces generated by the thermalmismatch are so extreme that it was found impractical to mechanicallysolve the heater failure problem by increasing the bolt strength.

The present invention uniquely solved the problem by changing thestructural features of the pump to exploit a physical phenomena existingin the operating pump. During pump operation there is a pressuredifferential across the base plate 12 because of the reduced pressureexisting within and the atmospheric pressure existing outside the pumphousing 11. This pressure differential produces an inward deflection ofthe base plate 12 in opposition to the outward spherical deflectioncaused by the heat flux. A base plate 12 thickness can be calculatedwhich under normal operating conditions will result in a certain baseplate pressure deflection. Accordingly a combined thermal and pressuredeflection can be established for the base plate 12 which issubstantially equal to the thermal deflection of the heater plate 16under normal pump operating conditions. In this way the stress on thesecuring bolts 18 can be reduced to a minimum and the above describedheater failure eliminated.

Matching the base plate and heater plate deflections for a given pumpentails calculation and use of a base plate bottom thickness such thatthermal deflection P of the base plate as restrained by the attachedpump housing plus the pressure deflection 5 of the restrained base platewill substantially equal the thermal deflection P of the unrestrainedheater plate with the pump operating at rated pressure differential APacross the base plate and heat flux Q across the interface betweenheater plate and base plate. Under these conditions the stress on therestraining bolts will be substantially zero and the failure problemseliminated.

I Some degree of base plate pressure defection will obviously existregardless of its thickness. Practically, however a substantial degreeof pressure defection must be provided if a real thermal deflectionmismatch is to be solved. For example, if the thermal deflectioninequality is significant a pressure deflection of at least 1.4 10-times the diameter of the circular base plate must be provided if themismatch is to be adequately compensated. This magnitude of pressuredeflection is measured at the center of the circular base plate.

The restoring moment exerted on the plate 12 by the pump housing 11 inmany pump embodiments will be negligible. This is especially true wherea thermal gradient exists in the pump housing 11 during operation of thepump. Such a gradient tends to reduce any restoring moment exerted bythe pump housing 11. In such instances the above equalization of heaterplate and base plate deflections can be obtained by calculating a baseplate thickness in accordance with the following equation:

2 II QLEZ 2Em Q k2 where AP represents the normal operating pressuredifferential for which the pump is \designed across the base plate 12 inpounds per square inch (p.s.i.), E represents the modulus of elasticityof the base plate 12 in p.s.i., m represents the inverse of Poissonsratio, a represents the outside radius of the base plate 12 in inches, Qrepresents the normal operating heat flux in B.t.u.s per hour per squareft. (B.t.u./hr.-ft. across the interface between the heater plate 16 andthe base plate 12, a represents the thermal expansion coefficient of thebase plate 12 in inches per inch per F. (in./in./ F.), k represents theconductivity of the base plate 12 in B.t.u.s per foot per hour persquare foot per F. (B.t.u.-ft./hr.-ft. F.), 00 represents the thermalexpansion coefficient of the heater plate 16 in in./in./ F. and krepresents the thermal conductivity of the heater plate 16 inB.t.u.-ft./hr.-ft. F. Tests with such pumps have shown that use of baseplate thickness 10% greater than indicated by the above formula willresult in a significant incidence of heater failure.

Another important characteristic of the invention is the provision of asecuring nut 17 and a threaded stud 18 through the center of the heaterplate 16. This feature permits the operation of a pump designed asdescribed above at lower than rated heat flux. Under such a condi tionthe pressure deflection 6 of the base plate 12 will more than compensatefor the difference in thermal deflections P P of the base plate 12 andthe heater plate 16.

Thus an undesired separation gap will tend to form at the center ofthese elements. However, the restraining force of the centrally locatednut and stud will prevent the formation of this gap. It will beunderstood that the mechanical force necessary to prevent thisseparation is substantially less than that required to prevent thethermally induced circumferential separation described above. For thisreason the use of conventional bolting techniques are adequate.

Because the deflection caused by the pressure gradient across the baseplate 12 inherently can be obtained in only one direction the presentinvention is of particular value in a diffusion vacuum pump wherein thetemperature induced deflection of the base plate 12 is greater than thatof the heater plate 16. This condition will exist if the 6/ k of thebase plate material is greater than that of the heater plate materialwhere 6 represents the thermal expansion coefficient in in./in./ F. andk represents the thermal conductivity in B.t.u.-ft./hr.-ft. F.

It will also be recognized that the problem solved and therefore thesolution itself is uniquely related to diffusion vacuum pumps adapted tooperate with a relatively high heat flux across the interface betweenthe heater plate and the base plate. By relatively high is meant a heatflux greater than 15,000 B.t.u./hr.-ft. Such high heat fluxes have comeinto extensive use only recently with the advent of diffusion pumpsdesigned to provide increased performance characteristics.

Thus, the present invention provides a much improved diffusion vacuumpump having high performance capabilities and exhibiting a strongresistance to heater failure.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is, therefore, to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. A diffusion vacuum pump apparatus comprising a cylindrical pumphousing adapted for connection with a chamber to be evacuated, acircular substantially uniform thickness base plate composed of acertain heat conducting metal and gas tightly sealed to said cylindricalpump housing so as to close the bottom end thereof forming a pump boilerportion adapted to contain a pumping fluid, a heater plate composed of aheat conducting metal dissimilar in composition to said certain metaland attached to the exterior of said pump housing in close contact withsaid circular base plate, said heater plate being adapted to supply heatenergy through said circular base plate to the pumping fluid in saidpump boiler portion, and wherein the thickness of said circular baseplate is such that under normal operating conditions the designed normaloperating pressure differential existing across said circular base platewill cause a spherical deflection thereof which deflection as measuredby the total linear deflection perpendicular to and at the center ofsaid circular base plate will have a magnitude of at least 1.4 10 dwhere d represents the diameter of the circular base plate.

2. A diffusion vacuum pump apparatus according to claim 1 wherein the6/k of said base plate material is greater than that of said heaterplate material where 5 represents thermal expansion coeflicient inin./in/ F. and k represents thermal conductivity in B.t.u.-ft./hr.- ft.F.

3. A diffusion vacuum pump apparatus according to claim 2 wherein saidheater plate comprises a circular block having a surface interrupted byindentations and electrically conductive means positioned in andconforming to said indentations so as to be in close contact with saidcircular block.

4. A diffusion vacuum pump apparatus according to claim 1 wherein saidheater plate comprises a circular block having a surface interrupted byindentations and electrically conductive means positioned in andconforming to said indentations so as to be in close contact with saidcircular block.

5. A diffusion vacuum pump according to claim 1 in cluding bolt meansacting at the center of said circular base plate and said heater plateto restrain relative parting movement between the center portionsthereof.

6. A diffusion vacuum pump apparatus comprising a cylindrical pumphousing adapted for connection with a chamber to be evacuated, a baseplate composed of stainless steel and gas-tightly sealed to saidcylindrical pump housing so as to close the bottom end thereof forming apump boiler portion adapted to contain a pumping fluid, a heater platedefined by a circular iron block having a surface interrupted byindentations and having electrical- 1y conductive means positioned inand conforming to said indentations so as to be in close contact withsaid iron block, said heater plate being attached to the exterior ofsaid pump housing in close contact with said base plate, said heaterplate being adapted to supply heat energy through said base plate to thepumping fluid in said pump boiler portion, and wherein the mechanicalcharacteristics of said cylindrical housing and attached base plate andthe mechanical characteristics of said heater plate are such that 6 +Pwill substantially equal P with said diffusion vacuum pump operating atrated AP and Q", where P represents the thermal deflection of said baseplate as restrained by said attached cylindrical housing, P representsthe unrestrained thermal deflection of said heater plate, 6 representsthe pressure deflection of said base plate as restrained by saidcylindrical housing, AP represents the designed normal operatingpressure differential across said base plate and Q presents the designednormal operating heat flux across the interface between said heaterplate and said base plate.

7. A diffusion vacuum pump apparatus comprising a cylindrical pumphousing adapted for connection with a chamber to be evacuated, a baseplate composed of a certain heat conducting material and gas tightlysealed to said cylindrical pump housing so as to close the bottom endthereof forming a pump boiler portion adapted to contain a pumpingfluid, a heater plate composed of a material dissimilar in compositionto said certain material and attached to the exterior of said pumphousing in close contact with said base plate, said heater plate beingadapted to supply heat energy through said base plate to the pumpingfluid in said pump boiler portion, wherein the mechanicalcharacteristics of said cylindrical housing and attached base plate andthe mechanical characteristics of said heater plate are such that oq+Pwill substantially equal P with said diffusion vacuum pump operating atrated AP and Q where P represents the thermal deflection of said baseplate as restrained by said attached cylindrical housing, P representsthe unrestrained thermal deflection of said heater plate, 1x representsthe pressure deflection of said base plate as restrained by saidcylindrical housing, AP represents the designed normal operatingpressure differential across said base plate and Q" represents thedesigned normal operating heat flux across the interface between saidheater plate and said base plate, and wherein said base plate has auniform thickness in inches which is less than 2 H 9 Q [01 [6 where APis equal to the normal operating pressure difference across said baseplate in p.s.i., E is equal to the modulus of elasticity of said baseplate in p.s.i., m is equal to the inverse Poissons ratio, ais equal tothe outside radius of said base plate in inches, Q" is equal to thenormal operating heat flux in B.t.u./hr.-PT across the interface betweensaid heater plate and said base plate, 04 is equal to the thermalexpansion coeflicient of said base plate in in./in./ F., k is equal tothe thermal conductivity of said base plate'in B.t.u.-ft./hr.-ft. F., ais equal to the thermal expansion coefficient of said heater plate inin./in./ F., and k is equal to the thermal conductivity of said heaterplate in B.t.u.-ft./hr.-ft. F.

7 8 8. A diffusion vacuum pump apparatus according to FOREIGN PATENTSclaim 7 wherein the oz/k, of said base plate material is 449 820 7/1948Canada greater than that of said heater plate material where at 6:8193/1956 Germany represents thermal expansion coefiicient in in./in./ F.

and k represents thermal conductivity in B.t.u.-ft./ OTHER REFERENCEShr.-ft. F. 0 National Research Corporation Technical Bulletin ReferencesCited 0100-02, Dilfusion Pumps and Ultra-High Vacuum, pp. UNITED STATESPATENTS 045,098 7/1962 Norton 219538 X 10 RICHARD M. WOOD, PrimaryExaminer. 1879212 9/1932 Hamlen c. L. ALBRITTON, Assistant Examiner,2,447,636 8/1948 Colaiaco 230101 I 2,668,005 2/1954 Wishart 2301 01 U S-C1, X R

29,03,181 9/1959 GiePen 230401 16564;219538,548;230101,"238; 23678

